This invention relates generally to cable modems, and more particularly to an upstream programmable gain amplifier.
Cable modems are being deployed today that allow high-speed Internet access in the home over a cable network, often referred to as a hybrid fiber copper (HFC) cable network. The architecture of a typical cable modem used in a cable network is shown in FIG. 1. A cable modem 10 is a unit, often referred to as consumer premise equipment (CPE), that is connected to a personal computer (PC) or other computing device, for example. The cable modem 10 is adapted to communicate with the cable modem termination system (CMTS) 18 that is typically located at a cable network provider""s headend. The cable modem 10 is a modulator/demodulator that receives Internet traffic or other information from a server through the CMTS 18 and puts it into a format recognizable by a user""s PC, allowing a user to browse the Internet and send/receive e-mail just as they would with a conventional modem on a PC. Using a cable modem 10 over a cable network provides a much faster connection, being at least 50 times faster than a 56K modem, for example.
Cable modem 10 includes a cable modem integrated circuit (IC) 12 which may comprise a single IC or a plurality of IC""s. The cable modem IC 12 performs the modulation and demodulation for the cable modem 10 and the operations necessary to interface with the PC. Cable modem 10 also includes support circuitry such as memory elements and other components, not shown. Cable modem 10 includes a tuner 16 coupled to cable modem IC 12 through a power amplifier that is referred to as a programmable gain amplifier (PGA). Tuner 16 interfaces the cable modem 10 with the CMTS 18 of the cable network and handles the upstream and downstream signal paths or data streams. PGA 14 drives the upstream direction signal through the tuner 16. The PGA 14 interfaces the cable modem IC 12 to the cable network CMTS 18. The PGA 14 drives the upstream data signals on the cable that is plugged into the cable outlet in a home, and the cable is coupled to the headend or CMTS 16. PGA 14 may be packaged as part of the tuner 16, or alternatively, the PGA 14 may be packaged separately from the tuner 16 within the cable modem 10, as shown. The programmable gain amplifier (PGA) 14 provides the power amplification of the upstream channel.
In the United States and other countries, the cable industry is guided by the Data Over Cable System Interface Specification (DOCSIS) standard. According to DOCSIS, the CMTS 18 expects to receive a signal having a particular signal power level from every cable modem 10. Because the distance between the cable modem 10 user and the CMTS 18 module varies from home to home, cable modem 10 is required to transmit differing power levels in order to ensure that the same cable modem upstream signal power level is received at the CMTS 18 for all cable modems 10 serviced by the CMTS 18. The PGA 14 is responsible for providing the variable programmable gain for the upstream signals from the cable modem 10 to the CMTS 18 to achieve differing power levels for different homes depending on how far away they are from the CMTS 18.
The DOCSIS standard places requirements on the characteristics of the cable modem 10 signal output which the PGA 14 is responsible for amplifying. These DOCSIS requirements include certain system performance levels, such as in-band and out-of-band frequency harmonics. The CMTS 18 tells the cable modem 10 what frequency to transmit on, which is basically what channel to use, in the upstream direction. The cable network frequency range may be, for example, from 5 megahertz to 65 megahertz in the European standard, and 5 megahertz to 42 megahertz in the North American standard.
Certain elements of the cable modem 10 such as the digital-to-analog converter (DAC) (not shown) of the cable modem IC 12 or the power amplifier may generate harmonics, which are multiples of the channel frequency. These harmonics may interfere with other user""s cable modems, or the other signals coming from other users"" cable modems that may be assigned different channels. Sometimes the upstream harmonics can fold into in the downstream signal, causing interference. DOCSIS places certain levels on those harmonics to ensure that they are at a sufficiently low power level. The distortion performance of the PGA 14 is important in that regard.
A variety of different architectures have been used for prior art cable modem PGA""s 14, which typically comprise Class A and other power amplifiers and have a variable programmable gain. A problem with prior art PGA""s 14 is that their power dissipation is invariant with the gain settings, resulting in an inefficient use of power in the cable modem 10.
The present invention optimizes system power of a cable modem by providing a PGA having a programmable bias current. The direct relationship of distortion performance to bias current within a Class A amplifier is used advantageously to provide programming capability of the PGA bias current. The programmable bias current PGA includes a bias current-setting circuit coupled to the bias input of a power amplifier stage.
Disclosed is an upstream programmable gain amplifier (PGA) for a cable modem, comprising a power amplifier stage and a bias current-setting circuit coupled to the power amplifier stage bias input, where the bias current-setting circuit is adapted to program the bias current of the power amplifier stage.
Also disclosed is a bias current-setting circuit for an upstream programmable gain amplifier (PGA) of a cable modem, the circuit including a bandgap generator coupled to an input of the variable gain amplifier and an external resistor coupled to an input of the bandgap generator.
Further disclosed is a cable modem for transmitting data signals between a cable modem termination system (CMTS) over a cable network, the cable modem comprising a cable modem integrated circuit (IC) adapted to receive upstream data signals from a personal computing device and a programmable gain amplifier (PGA) coupled to the cable modem IC output having a programmable bias current and adapted to amplify the upstream data signals. A tuner is coupled to the PGA output for modulating and demodulating the amplified upstream data signals in preparation for transmission to the CMTS.
Also disclosed is a method of programming the bias current of a programmable gain amplifier for a cable modem, comprising establishing a nominal bias current, receiving a bias code, using the bias code to vary the nominal bias current to produce a bias current for a programmable gain amplifier, and applying the bias current to the programmable gain amplifier bias input.
Advantages of the invention include increasing the efficiency of a cable modem and PGA by optimizing the power for a given required level of performance. Power versus the required performance and the bias current versus a bias code may be varied. The power savings is particularly beneficial for voice-over IP and high-speed data service applications for cable modem systems. Gain may be varied in 6 dB steps, which is coarser than required by DOCSIS. The invention is inexpensive to implement, requiring few circuit components and a small amount of memory.